A 32-year-old man is admitted to the emergency department with visual problems. Radiographic examination reveals a tumor of the adenohypophysis (anterior pituitary gland). Physical examination reveals a loss of the lateral halves of the fields of vision of both eyes (bitemporal hemianopia or “tunnel vision”). Which of the following structures was most likely compressed by the tumor?

**Core Concept**
The question tests understanding of the anatomical relationship between the anterior pituitary gland and surrounding structures, particularly those involved in vision. The **adenohypophysis** is located near critical visual pathways.

**Why the Correct Answer is Right**
The clinical presentation of bitemporal hemianopia suggests compression of the **optic chiasm**, where fibers from the nasal hemiretinas of both eyes cross. A tumor of the adenohypophysis can compress the optic chiasm, leading to this specific visual field defect. The optic chiasm is a critical structure for transmitting visual information.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because, while the **optic nerve** is involved in vision, compression of one optic nerve would result in unilateral blindness, not bitemporal hemianopia.
**Option B:** Incorrect as the **oculomotor nerve** primarily controls eye movement, not vision.
**Option D:** Incorrect because the **pituitary stalk** connects the hypothalamus to the pituitary gland and is not directly involved in transmitting visual information.

**Clinical Pearl / High-Yield Fact**
Remember, bitemporal hemianopia is a classic sign of a lesion affecting the **optic chiasm**, often due to a pituitary tumor. This visual defect is a critical clue for diagnosing such conditions.

**Correct Answer:** C. optic chiasm

✓ Correct Answer: B. Optic chiasm